Combustible gas detection system

ABSTRACT

A system is provided for monitoring the levels of combustible gas in a gas stream. The system includes means for controlling the relative humidity of the gas stream and maintain a humidity level in the performance range of combustible gas sensors. A number of methods are illustrated for achieving the humidity control including secondary phase separations and the adjusting of the gas stream temperature.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/707,324 which was filed on Dec. 5, 2003

FIELD OF THE INVENTION

This disclosure relates generally to the detection of combustible gases,and especially relates to the detection of hydrogen in a vent gasstream.

BACKGROUND OF THE INVENTION

Hydrogen gas is used and produced in many applications. Since the amountof hydrogen in a gas stream produced by a given process may be anindicator of system efficiency, the systems typically utilize sensors,such as combustible gas sensors to determine the level of hydrogen. Anexample of a prior art system having an arrangement for monitoringcombustible gas is shown in FIG. 1A. The electrochemical system 12receives water from an external source 14 and passes it through adeionizing bed 16. Once the water has been properly conditioned, it issupplied to an electrochemical cell 18 which disassociates the waterinto hydrogen and oxygen gas.

One example of an electrochemical cell 18 is a proton exchange membraneelectrolysis cell which can function as a hydrogen generator byelectrolytically decomposing water to produce hydrogen and oxygen gas,and can function as a fuel cell by electrochemically reacting hydrogenwith oxygen to generate electricity. Referring to FIG. 1B, which is apartial section of a typical anode feed electrolysis cell 100,conditioned water 102 is fed into cell 100 on the side of an oxygenelectrode (anode) 116 to form oxygen gas 104, electrons, and hydrogenions (protons) 106. The reaction is facilitated by the positive terminalof a power source 120 electrically connected to anode 116 and thenegative terminal of power source 120 connected to a hydrogen electrode(cathode) 114. The oxygen gas 103 and a portion of the process water 108exit cell 100, while protons 106 and water 110 migrate across a protonexchange membrane 118 to cathode 114. At cathode 114, hydrogen gas 112is formed and removed. Water is also removed from cathode 114.

A typical fuel cell uses the same general configuration as is shown inFIG. 1B. Hydrogen gas is introduced to the hydrogen electrode (the anodein fuel cells), while oxygen, or an oxygen-containing gas such as air,is introduced to the oxygen electrode (the cathode in fuel cells). Watercan also be introduced with the feed gas. The hydrogen gas for fuel celloperation can originate from a pure hydrogen source, hydrocarbon,methanol, or any other hydrogen source that supplies hydrogen at apurity suitable for fuel cell operation (i.e., a purity that does notpoison the catalyst or interfere with cell operation). Hydrogen gaselectrochemically reacts at the anode to produce protons and electrons,wherein the electrons flow from the anode through an electricallyconnected external load, and the protons migrate through the membrane tothe cathode. At the cathode, the protons and electrons react with oxygento form water, which additionally includes any feed water that isdragged through the membrane to the cathode. The electrical potentialacross the anode and cathode can be exploited to power an external load.

In other embodiments, one or more electrochemical cells can be usedwithin a system to both electrolyze water to produce hydrogen andoxygen, and to produce electricity by converting hydrogen and oxygenback into water as needed. Such systems are commonly referred to asregenerative fuel cell systems.

After the electrochemical cell 18 disassociates the water, oxygen andhydrogen gas exit the cell 18 through conduits 20 and 22 respectively.As mentioned herein above, in addition to the gas products, waterentrained in the gases exits with the oxygen and hydrogen. The hydrogenconduit 22 typically connects with a hydrogen phase separator 24 whichextracts most of the water from the gas, with the water exiting thephase separator 24 through a valving arrangement which recycles thewater back into the electrochemical cell water feed conduit. Dependingon the needs of the application, additional water may be removed fromthe hydrogen gas by passing through an optional desiccant gas dryer 26before exiting the process for use in the application.

The oxygen gas stream 20 also enters into a phase separator 28 with amajority of the water separating from the gas stream and dropping to thebottom of the separator 28. As with the hydrogen separator 24 this wateris removed and recycled into the electrochemical cell water feedconduit. The separated hydrogen gas exits the phase separator 28 via aconduit 32 to exit the process. Since it is desirable to monitor for thepresence of hydrogen gas in the oxygen gas stream through an orifice 40to a combustible gas sensor 36. A gas dryer 38, such as a NAFION tubedryer, is usually placed in line between the phase separator 28 and thesensor 36 to remove water still entrained in the gas. Unfortunately,since the gas stream can still have a relative humidity greater than95%. This high relative humidity results in lower monitoring performancethan is desired.

Accordingly, what is needed in the art is a system for monitoringcombustible gas levels in a gas stream that reduces or eliminates theeffects of relative humidity on the combustible gas sensor.

SUMMARY OF INVENTION

A system for monitoring combustible gas includes a phase separatorhaving a first outlet, a conduit having an inlet connected to said phaseseparator and an exhaust outlet, and a combustible gas sensor adjacentsaid exhaust outlet and connected to said conduit. The combustible gassensor is generally mounted perpendicular to either the gas streamexhaust or the gas inlet exhaust. The conduit is generally made of ametal composition or from a conductive polymer.

An alternate embodiment of the system for monitoring combustible gasincludes a bracket having a main body and a first flange on one end ofsaid body and a second flange on an end of said body opposite said firstflange. A housing is mounted to the bracket, and the housing has anopening to receive a gas stream. A combustible gas sensor mounted to thefirst flange.

A system for generating hydrogen gas includes an electrochemical cellstack having a phase separator fluidly coupled to the electrochemicalstack for receiving a water gas mixture. A vent conduit fluidlyconnected and extending vertically from the top of the phase separator,and a combustible gas sensor coupled to the vent conduit.

BRIEF DESCRIPTION OF DRAWINGS

Referring now to the drawings, which are meant to be exemplary and notlimiting, and wherein like elements are numbered alike:

FIG. 1A is a schematic drawing of an electrochemical system having acombustible gas detection system used in the prior art;

FIG. 1B is a schematic diagram of a partial prior art electrochemicalcell showing an electrochemical reaction;

FIG. 2 is an illustration of an exemplary embodiment of a oxygen-waterphase separator having a combustable gas detector incorporated into thegas vent stream;

FIG. 3 is an illustration of an alternate embodiment of a combustiblegas sensor arrangement;

FIG. 4 is a simplified cross-section view of the of the combustible gassensor arrangement of FIG. 3;

FIG. 5 is an exploded view of the assembly of the combustible gas sensorarrangement of FIG. 3.

DETAILED DESCRIPTION

Hydrogen gas is a versatile material having many uses in industrial andenergy application ranging from the production of ammonia, to powervehicles being propelled into space. Since the hydrogen molecule is oneof the smallest known particles, containing and controlling leaks ofhydrogen gas is very difficult. Monitoring of these leaks is importantas it is typically an indicator of performance degradation and orcomponent wear. Typically, prior art systems have used combustible gassensors to monitor levels of combustible gas in the system. Whenunacceptable levels of hydrogen are detected in the system, the systemis either shut down, or the operator is alerted that preventativemaintenance is required.

Commercial combustible gas sensors typically use a technology referredto as a “catalytic bead” type sensor, such as the Detcon, Inc. ModelFP-524C. These sensors monitor the percentage of lower explosive limit(“LEL”) of combustible gas in a product gas stream. This LEL measurementrepresents the percentage of a combustible gas, such as hydrogen,propane, natural gas, in a given volume of air. One limitation ofcatalytic bead sensors is their sensitivity to moisture in the gas theyare monitoring. Once the gas reaches 95% relative humidity, the abilityof the sensor to detect combustible gas deteriorates resulting in lessthan desirable life and reliability. Many hydrogen applications,including but not limited to electrochemical cells, electrolyzers, fuelcells and methane steam reformers, also utilize water in their processeswhich tends to effect the relative humidity of the product gas streambeing monitored. It should be appreciated that while the examplesdescribed herein typically refer to electrochemical systems such aselectrolyzers or fuel cells, the present invention can be equallyapplied in any application where a combustible gas needs to bemonitored.

Referring to FIGS. 1A and 1B, and electrochemical system 12 of thepresent invention is shown. Electrochemical cells 18 typically includeone or more individual cells arranged in a stack, with the workingfluids directed through the cells within the stack structure. The cellswithin the stack are sequentially arranged, each including a cathode,proton exchange membrane, and an anode (hereinafter “membrane electrodeassembly”, or “MEA” 119) as shown in FIG. 1B. Each cell typicallyfurther comprises a first flow field in fluid communication with thecathode and a second flow field in fluid communication with the anode.The MEA 119 may be supported on either or both sides by screen packs orbipolar plates disposed within the flow fields, and which may beconfigured to facilitate membrane hydration and/or fluid movement to andfrom the MEA 119.

Membrane 118 comprises electrolytes that are preferably solids or gelsunder the operating conditions of the electrochemical cell. Usefulmaterials include, for example, proton conducting ionomers and ionexchange resins. Useful proton conducting ionomers include complexescomprising an alkali metal salt, alkali earth metal salt, a protonicacid, a protonic acid salt or mixtures comprising one or more of theforegoing complexes. Counter-ions useful in the above salts includehalogen ion, perchloric ion, thiocyanate ion, trifluoromethane sulfonicion, borofuoric ion, and the like. Representative examples of such saltsinclude, but are not limited to, lithium fluoride, sodium iodide,lithium iodide, lithium perchlorate, sodium thiocyanate, lithiumtrifluoromethane sulfonate, lithium borofluoride, lithiumhexafluorophosphate, phosphoric acid, sulfuric acid, trifluoromethanesulfonic acid, and the like. The alkali metal salt, alkali earth metalsalt, protonic acid, or protonic acid salt can be complexed with one ormore polar polymers such as a polyether, polyester, or polyimide, orwith a network or cross-linked polymer containing the above polarpolymer as a segment. Useful polyethers include polyoxyalkylenes, suchas polyethylene glycol, polyethylene glycol monoether, and polyethyleneglycol diether; copolymers of at least one of these polyethers, such aspoly(oxyethylene-co-oxypropylene) glycol,poly(oxyethylene-co-oxypropylene) glycol monoether, andpoly(oxyethylene-co-oxypropylene) glycol diether; condensation productsof ethylenediamine with the above polyoxyalkylenesl; and esters, such asphosphoric acid esters, aliphatic carboxylic acid esters or aromaticcarboxylic acid esters of the above polyoxyalkylenes. Copolymers of,e.g., polyethylene glycol monoethyl ether with methacrylic acid exhibitsufficient ionic conductivity to be useful.

Ion-exchange resins useful as proton conducting materials includehydrocarbon and fluorocarbon-type resins. Hydrocarbon-type ion-exchangeresins include phenolic resins, condensation resins such asphenol-formaldehyde, polystyrene, styrene-divinyl benzene copolymers,styrene-butadiene copolymers, styrene,styrene-divinylbenzene-vinylchloride terpolymers, and the like, that canbe imbued with cation-exchange ability by sulfonation, or can be imbuedwith anion-exchange ability by chloromethylation followed by conversionto the corresponding quaternary-amine.

Fluorocarbon-type ion-exchange resins can include, for example, hydratesof tetrafluoroethylene-perfluorosulfonyl ethoxyvinyl ether ortetrafluoroethylene-hydroxylated (perfluorovinylether) copolymers andthe like. When oxidation and or acid resist is desirable, for instance,at the cathode of a fuel cell, fluorocarbon-type resins having sulfonic,carboxylic and/or phosophoric acid functionality are preferred.Fluorocarbon-type resins typically exhibit excellent resistance tooxidation by halogen, strong acids, and bases. One family offluorocarbon-type resins having sulfonic acid group functionality isNAFION®resins (commercially available from E.I. du Pont de Nemours andCompany, Wilmington, Del.).

Electrodes 114 and 116 comprise catalyst suitable for performing theneeded electrochemical reaction (i.e. electrolyzing water to producehydrogen and oxygen). Suitable electrodes comprise, but are not limitedto, platinum, palladium, rhodium, carbon, gold, tantalum, tungsten,ruthenium, iridium, osmium, and the like, as well as alloys andcombinations comprising one or more of the foregoing materials.Electrodes 114 and 116 can be formed on membrane 118, or may be layeredadjacent to, but in contact with or in ionic communication with,membrane 118.

Flow field members (not shown) and support membrane 118, allow thepassage of system fluids, and preferably are electrically conductive,and may be, for example, screen packs or bipolar plates. The screenpacks include one or more layers of perforated sheets or a woven meshformed from metal or strands. These screens typically comprise metals,for example, niobium, zirconium, tantalum, titanium, carbon steel,stainless steel, nickel, cobalt and the the like, as well as alloys andcombinations comprising one or more of the foregoing metals. Bipolarplates are commonly porous structures comprising fibrous carbon, orfibrous carbon impregnated with polytetrafluoroethylene or PTFE(commercially available under the trade name TEFLON® from E.I. du Pontde Nemours and Company).

After hydrogen and oxygen have been disassociated from the water, thehydrogen exits the electrochemical cell 18 as described herein above viathe separator 24 and an optional dryer 26. The oxygen gas and excessprocess water exit the electrochemical cell through a conduit 20 whichcarries the oxygen and water into a phase separator 50 and exits thesystem through exhaust outlet 54. It should be noted that while thephase separator 50 removes water from the gas stream, the oxygen gastypically exits the separator 50 in a saturated condition with arelative humidity in excess of 95%.

Since high relative humidity has undesirable effects, the presentinvention addresses these issues by either controlling the temperatureof the gas stream or by controlling the pressure of the gas stream.Referring to FIGS. 2-5, two different types of combustible gas sensorarrangements are shown. As will be described in more detail herein, thearrangement of the gas sensor in combination with other componentsreduce the relative humidity of the sampled gas to increase theperformance of combustible gas measurements.

The combustible gas (“CG”) sensor arrangement utilized by the prior artis shown in FIG. 1A. In this arrangement, the CG sensor device 36includes a CG sensor 42 and a housing 44. The housing 44 is typicallytubular in shape and attaches to the sensor 42 by any convenient meanssuch as a thread (not shown). The CG sensor 42 also includes a sensingface 43 which detects the levels of combustible gas, this face 43 islocated opposite a housing open end 46. A gas sample tube 48 is insertedinto the open end 46. During operation, the saturated gas stream 49exits the sample tube 48 and mixes with the air in the housing allowingsome drying of the saturated gas.

An exemplary embodiment of the CG sensor of the present invention isshown in FIG. 2. In this embodiment, the CG sensor 36 is mounted to oneend of a vent conduit 52 adjacent a vent exhaust 54. The conduit 52 isvertically connected to above a water-gas phase separator 50. Theseparator 50 is a large container which receives water from an upstreamprocess such as an electrochemical cell 18 through tubes 56, 58. Theseparator may also utilize other components such as filters 60, waterlines 62, level sensors 64, and overflow drain 66.

In operation, the separator 50 receives the process water which maycontain entrained gases, including oxygen and possibly combustible gas,from tube 58. As the water mixture enters the separator 50 itexperiences a slight pressure drop causing some of the water entrancedin the stream to condense and drop to the bottom of the phase separator.The separated water exits via a conduit 68 to be either recycled backinto the process or is otherwise disposed of. The liberated gases, exitthrough conduit 52 and exit the system through exhaust outlet 54. As thegas vertically ascends conduit 52, additional water is separated fromthe gas stream through condensation on the side walls of conduit 52. Inthe preferred embodiment, the conduit 52 is made from a metal such asstainless steel to enhance the condensation of water out of the gas. Byknowing the operating conditions of the process and the temperature ofthe environment, the conduit 52 may be sized appropriately to dry thegas to desired relative humidity level to allow the CG sensor 36 tofunction as desired. A conductive metallic conduit 52 also providesadditional benefit in providing an electrical ground for the sensor 36.It should be noted that the electrical grounding provides a furtherbenefit of eliminating a possible voltage potential between the sensorand the conduit. By eliminating the voltage potential, the possibilityof an electrical arc forming between the sensor 36 and the conduit 52 isalso eliminated, which is advantageous when operating in an environmentwhich may contain combustible gases. Alternatively to the metallicconduit, a conductive polymer could also be used to achieve theappropriate grounding. The CG sensor 36 being positioned adjacent andperpendicular to the exhaust port 54 allows the monitoring of the gas toensure that any combustible gases present are maintained at appropriatelevels.

An alternate embodiment of combustible gas sensing arrangement is shownin FIGS. 3-5. This embodiment comprises a sensor assembly 70 having ahousing 76 secured to a bracket 72 by a pair of fasteners 90 whichthread into corresponding holes 92. A set of tabs 82 in housing 76 aresized and positioned to fit into corresponding slots 84 in the bracket72. To connect the vent conduit 52 to the assembly 70 a coupling 88secures the conduit 52 to a hole 86 in the housing 76. The housing 76further includes projections 94 on one end which provide for venting ofthe enclosed space created by the assembly. To provide for flexibilityin manufacturing of the assembly 70, the bracket 72 includes a firstflange 78 and second flange 79 for the mounting of the CG sensor 42. CGsensors 42 from different manufacturers may be of different sizes. Toaccommodate this variation, the CG sensor mounting holes 96 and 97 areof different sizes. To switch from one CG sensor manufacturer to anothersimply requires the bracket 72 to be rotated 180°, orienting the flange79 on top and mounting the CG sensor 42 to the flange 79. A cable 98connected to CG sensor 42 carries signals generated by the sensor 42 toa monitoring unit 99.

In this embodiment, which may be preferred in applications where avertical conduit is undesirable, the conduit 52 is connected to a sensorassembly 70 by coupling 88. As best shown in FIG. 4, the oxygen gasstream enters the assembly 70 through a housing 76 and impinges onbracket 72. As with the phase separator 50, as the stream enters theassembly 70, it experiences a further pressure drop which causes therelative humidity to less than 95%. The dried gas and any water exitthrough the open bottom portion 74. Due to the mixing of the gas streamwithin the assembly 70 when the stream contacts the bracket 72, the CGsensor 36 is able to monitor for levels of combustible gas. By arrangingthe sensor vertically above the entrance of the gas stream, the sensor42 can be protected from liquids in the stream and providing a drier gasfor monitoring. Since combustible gases such as hydrogen are lighterthan air, any hydrogen mixed with the oxygen gas stream will dispersevertically toward the CG sensor 42, to prevent the accumulation ofcombustible gases in the assembly 70 which would result in faultymeasurements, a set of vent openings formed between the housing and theflange 78 by the projections 94 adjacent to the CG sensor 42.

It should be appreciated that the flanges 78, 79 for mounting the CGsensor 42 may alternatively be located on the housing 76. Additionaladvantages in calibration of the sensor 42 are achieved by positioningthe flanges 78, 79 as shown in the preferred embodiment. CG sensors suchas those which are described herein require a periodic calibration toensure proper measurements. These calibration procedures typicallyinvolve using a canister of premixed combustible gas having apredetermined LEL and introducing the gas to the sensor. For accurateresults to be achieved, the premixed gas must be introduced directlyadjacent the sensor. To calibrate the system as shown in the preferredembodiment, the user simply needs to remove the housing 76 by removingbolts 90 without disturbing the CG sensor. The premixed gas can then beintroduced to the sensor 42 without any physical hindrances to theprocedure.

While preferred embodiments have been shown and described, variousmodifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention. For example, while theembodiments shown referred specifically to an electrochemical systemgenerating hydrogen, this invention would apply equally to any systemwhere there is a potential for mixing hydrogen with air or oxygenincluding, but not limited to photolysis, fuel cells, steam methanereformers or hydrocarbon reformers. Accordingly, it is to be understoodthat the present invention has been described by way of illustrationsand not limitation.

1. A system for generating hydrogen gas comprising: an electrochemicalcell stack; a phase separator fluidly coupled to said electrochemicalstack for receiving a water gas mixture; a vent conduit fluidlyconnected and extending vertically from the top of said phase separator;and, a combustible gas sensor coupled to said vent conduit.
 2. Thesystem for generating hydrogen gas of claim 1 wherein said vent conduitis metallic.
 3. The system for generating hydrogen gas of claim 2wherein said combustible gas sensor is electrically grounded to saidvent conduit.
 4. The system for generating hydrogen gas of claim 3wherein said vent conduit further comprises an exhaust outlet.
 5. Thesystem for generating hydrogen gas of claim 4 wherein said combustiblegas sensor is positioned adjacent to said exhaust outlet.
 6. A systemfor generating hydrogen gas comprising: an electrochemical cell stackhaving and anode and electrode, said electrochemical cell stack furtherhaving at least one gas outlet and a means for electrolyticallydecomposing water to produce hydrogen gas; a phase separator having abottom and a top portion thereon, said phase separator being fluidlycoupled to said electrochemical stack gas outlet for receiving a watergas mixture; a vent conduit fluidly connected and extending from saidphase separator top portion; and, a catalytic bead type combustible gassensor coupled to said vent conduit, said combustible gas sensor havingsensing means for determining the lower explosive limit of gas withinsaid vent conduit.
 7. The system for generating hydrogen gas of claim 6wherein said vent conduit comprises a body having a inlet fluidlycoupled to said phase separator top portion, an outlet positioned at abody first end and perpendicular to said inlet, and a vent slot beingpositioned at a body second end opposite said outlet.
 8. The system forgenerating hydrogen gas of claim 7 wherein said body is comprised of ahousing portion mounted to a bracket portion, said bracket portionhaving a first and second flange wherein said body outlet is positionedin said first flange.
 9. The system for generating hydrogen gas of claim8 wherein said body is made from stainless steel.
 10. The system forgenerating hydrogen gas of claim 7 wherein said combustible gas sensoris electrically connected to said vent conduit.
 11. The system forgenerating hydrogen gas of claim 10 wherein said electrical connectionis an electrical ground.
 12. The system for generating hydrogen gas ofclaim 6 wherein said vent conduit further comprises an exhaust outlet.13. The system for generating hydrogen gas of claim 12 wherein saidsensing means is positioned adjacent to said exhaust outlet.
 14. Asystem for generating hydrogen gas comprising: a water conduit; anelectrochemical cell stack having an inlet connected to said waterconduit and a gas outlet, said cell stack further having a membraneelectrode assembly comprising an anode and cathode with a solid membranearranged in between, said membrane electrode assembly being arranged toallow electrolytic decomposion of water received from said water conduitto produce hydrogen gas wherein said hydrogen exits said gas outlet; aphase separator having an inlet coupled to said cell stack outlet and awater outlet fluidly coupled to said water conduit, said phase separatorfurther having a vent outlet arranged opposite said water outlet; a ventconduit fluidly connected and extending from said phase separator ventoutlet, said vent conduit having an inlet and an outlet being arrangedperpendicular to said inlet; and, a combustible gas sensor having amonitoring unit and a sensor unit, said sensor unit being operablycoupled to said vent conduit.
 15. The system for generating hydrogen gasof claim 14 wherein said sensor unit is mounted to said vent conduitopposite said vent conduit outlet.
 16. The system for generatinghydrogen gas of claim 15 wherein said sensor unit is a catalytic beadtype sensor.
 17. The system for generating hydrogen gas of claim 15wherein said vent conduit is comprises: a housing having an openingcoupled to said phase separator vent outlet; and, a bracket coupled tosaid housing and having a wall portion positioned opposite to andgenerally perpendicular to said housing opening.
 18. The system forgenerating hydrogen gas of claim 17 wherein said housing has a pair oftabs extending therefrom, and said bracket has at least one pair ofslots sized and positioned to receive said housing tabs.
 19. The systemfor generating hydrogen gas of claim 18 wherein said bracket furtherincludes a first and second flange extending perpendicular to saidbracket wall portion, said first and second flanges being arranged onopposite ends of said bracket.
 20. The system for generating hydrogengas of claim 19 wherein said first flange has an opening sized toreceive said sensor unit.